Laminated rings for use in belts for continuously variable transmissions are manufactured by laminating a plurality of ring-shaped members having slightly different circumferential lengths. The ring-shaped members are prepared by welding opposite ends of a thin sheet of maraging steel, which is an ultrahigh strength steel, into a cylindrical drum, and slicing the cylindrical drum into thin-sheet metal rings. The thin-sheet metal rings are then rolled into ring-shaped members having respective desired circumferential lengths.
One known apparatus for rolling such metal rings is disclosed in Japanese laid-open patent publication No. 11-290908. The disclosed apparatus has a pair of tension rollers spaced horizontally a given distance from each other for supporting a thin-sheet metal ring thereon. The rolling apparatus also has a guide roller disposed intermediate between the tension rollers and a rolling roller for gripping and rolling the metal ring in coaction with the guide roller.
The rolling roller is pressed against the metal ring by a rolling cylinder. At least one of the tension rollers is displaceable away from the other tension roller by a tension cylinder.
The rolling apparatus operates as follows: The metal ring is trained around the tension rollers. The rolling cylinder is actuated to press the rolling roller against the metal ring, which is gripped between the rolling roller and the guide roller. The rolling roller is rotated to roll the metal ring to progressively increase the circumferential length of the metal ring. When the metal ring is rolled, the tension cylinder displaces at least one of the tension rollers away from the other tension roller by a distance corresponding to the increase in the circumferential length of the metal ring, thus tensioning the metal ring. The displacement of the tension roller prevents the metal ring from dropping off the tension rollers.
Then, the rolling apparatus measures the circumferential length of the metal ring which is progressively increased when the metal ring is rolled. When the circumferential length of the metal ring has reached a desired value, the rolling process is finished. The circumferential length of the metal ring can be determined as a function of the distance between the axes of the tension rollers. The rolling apparatus uses an encoder, for example, for measuring the distance by which the tension roller is displaced by the tension cylinder. The encoder detects a completion of the rolling process when the distance by which the tension roller is displaced reaches a given value. Then, the encoder outputs an electric signal to stop the tension cylinder and the rolling cylinder. After the rolling cylinder is thus stopped, it releases the rolling roller from pressing the metal ring.
The rolling cylinder takes a time ranging from 0.01 to 0.1 second in releasing the rolling roller from pressing the metal ring because of a mechanical device used to release the rolling roller. During such a time, the rolling roller remains pressed against the metal ring, and continuously rotates due to the inertia from the rolling process. As a result, the metal ring is excessively rolled after the rolling process is completed.
The tension cylinder and the rolling cylinder are separate mechanical arrangements which cause them to stop at different times, which are 0.01 to 0.1 second apart from each other, in response to the electric signal from the encoder. If the stoppage of the rolling cylinder is delayed due to the stop timing difference, then the metal ring may further be excessively rolled. Even if the rolling cylinder is stopped prior to the tension cylinder, the metal ring may further be excessively rolled because of continued rotation due to the inertia.
When the metal ring is excessively rolled after the rolling process is completed, the tension ring has already been stopped. Therefore, no tension is imparted to the metal ring which is excessively rolled. As a consequence, the metal ring may possibly fall off the tension rollers.